The present invention relates to information recording media for recording analog signals of images and sounds, which are frequency modulated, and digital information such as computer data, facsimile signals, and digital audio signals in real time by recording energy such as a laser beam, electron beam, and electric current.
There are a variety of principles for recording on the thin film by energy beam such laser beam, and because the recording by phase change between amorphous and crystalline materials, between crystalline and crystalline materials, and between amorphous and amorphous materials, recording by reactions between the upper and lower layers (alloying) and magneto-optical recording are hardly accompanied with changes of film configuration, their advantages are that a double sided disk comprising two disks pasted together is formed and that records can be rewritten.
However, the recording by phase change has the problem that the rewriting cyclability is restricted due to the flow of the recording film when a record is rewritten many times. This is because in melting the recording film for amorphization, the recording film side of the protecting layers on both sides of the recording film expands by heat and the opposite side does not expand by heat so much as the recording film so that the protecting film expands toward the recording film and consequently the recording film is pressed and flows toward the portion at a lower temperature. When recording laser power is particularly high, the recording film partially evaporates, so that the effect of pressing the recording film increases. Although the flow by a single recording is quite slight, if the same position on the recording track is melted repeatedly many times, the film thickness of that part decreases thereby causing a read-out error. Thinning the recording film as countermeasure for this problem has been proposed in the proceeding volume 1078 (1989) of Society of Photo Optical Instrumentation Engineers (SPIE), pp. 27-34 (hereinafter referred to as the first conventional technology). According to this method, by thinning the recording film, the heat capacity of the recording film is decreased so as to prevent the deformation of the upper and lower protective layers due to the rise of temperature and the flow speed of the recording film is reduced by the adhesive force of a melted recording film to the upper and lower protective layers and viscosity.
The production of less than 100 nm uneven surface by etching germanium (Ge) film by reactive ion has been publicized in Applied Physics Letter volume 40, No. 8, pp. 662-664 "Textured Germanium Optical Storage Medium" (hereinafter referred to as the second conventional technology) and U.S. Pat. No. 4,422,159 (hereinafter referred to as the third conventional technology). While only the recording by deformation of the recording film with uneven surface is described in the second conventional technology, the third conventional technology mentions the recording by oxidation of the recording film as well as the recording by deformation of the recording film.
Furthermore, a recording method that pit arrays closely filled in the recording track direction are formed on the plastic substrate surface of an optical disk, a mixture of fine metallic particles and paraffine are filled in each pit of the pit arrays and covered with a protective layer, and the distribution of fine particles is changed by applying a magnetic field and heating by a laser beam is indicated in U.S. Pat. No. 4,841,515.
However, although the first conventional technology has improved the rewriting cyclability as compared with relatively thick recording film, it cannot realize the unlimited multiple rewriting required when used as a computer file memory because it does not block the flow completely. The second and third conventional technologies are not able to rewrite recordings because recording is done by deformation or oxidation of the recording film, thus the application is limited. Additionally, the characteristic is likely to change due to the oxidation of the film surface and the service life for storage is short. This is because no protective film is formed or if any, organic protective film which oxygen is easy to pass and likely to deform by heat is formed on the uneven surface side. Thus the second and third conventional technologies do not guarantee multiple rewriting although they enable recording by thermal deformation or oxidation.
The fourth conventional technology has disadvantages that the reproduced signal quality is low because no recording material is formed on protrusions of the substrate and the protective layer is easy to separate because the surface of the protective layer on the recording film side is flat and hence little effective in preventing deformation of the recording film. Comparing with these technologies, this invention enables multiple rewriting because the flow of recording film due to energy beam radiation on the recording film can be prevented almost completely as inorganic protective film with a high melting point is provided on the uneven surface, and the protective film gets into the bents of the uneven surface.
Furthermore, the present invention realizes good reproduced signal quality because the signal level is high and the noise level is low.
On the other hand, magneto-optical recording produces a large diffusion of heat in horizontal direction (recording film inside direction) due to the high coefficient of thermal conductivity, so that a recording point faithful to recording signal is difficult to form. The axis of easy magnetization is difficult to be set completely at right angle to the film surface.